420 Dr. T. Williams on the Mechanism of Aquatic 



preserved in spirit, aided by the successful injections of Milne- 

 Edwards.* Haliotis tuherculata is a typical Scutibranch. The 

 branchiae (fig. 11 a, a) are lodged in a spacious enclosure of the 

 mantle on the left side of the body. They are attached at the base 

 by means of the vascular trunks ; on the inferior surface to some 

 extent of their length, by means of a membranous frenum. They 

 are doubly laminose; that is, between two parallel trunks 

 [b, b &, c, c), a, vein and an artery, which run from the base to 

 the apex of each gill, there lie transversely two series of leaves 

 {a, a', a, a') which are traversed by the blood of the afferent 

 vessel {b) on its path to the efferent vessel (c). Here again, as 

 in all the Gasteropod Mollusks, the gills are observed to conform 

 in general structure to those leafy piles which fill the thoracic 

 cavity of the Crab ; but how totally different in the minute parts 

 of their organization ! In Haliotis the heart (e) is systemic. The 

 blood, freshly arterialized, returns directly from the branchiae 

 into the auricles (d, d) ; from the ventricle {e) it is driven into 

 the systemic aorta (/). At the roots of the branchiae the 

 venous blood converges into capacious, but not lacunose, afferent 

 trunks [b, b), along which it is conducted to the branchial 

 laminae. The vascular system of the laminae (fig. 12) is the 

 same in every respect as that already described and figured in 

 Chiton and Patella. 



Each lamina carries two layers of horizontally stratified, uni- 

 formly diametered tubes, united to one another laterally, and to 

 those of the opposite venous stratum by transverse threads. 

 Each leaflet consequently consists of two layers or planes of 

 blood-channels, joined together at the free margin, and covered 

 on their external surfaces by a ciliated epithelium. A thin film of 

 water moves wavingly from the fixed to the free borders : it is 

 sustained in motion by the vibration of the cilia. Such an arrange- 

 ment proves that not only the blood to be aerated, but the water 

 which aerates, is required in the act of respiration to be divided 

 to the utmost degree. As far as this inquiry has proceeded, it 

 must have been observed, that although the plumose or pinnate 

 form of gill is constantly adopted as the most convenient figure 

 in accordance with which to construct the respiratory organs 

 both in Mollusca and Crustacea, the resemblance never proceeds 

 beyond the exterior conformation and the disposition of the 

 larger vessels. Beyond and deeper than this limit the micro- 

 scope establishes an irreconcilable difference between the Crus- 

 tacean and Molluscan gill. It exemplifies the supreme value of 

 this instrument in zoological research. But apart from these 

 physical considerations, a chemical ground of dissimilarity be- 



* Annales des Sciences Naturelles, torn. viii. 184/. 



